Coated inorganic fiber reinforcement materials and ceramic composites comprising the same

ABSTRACT

Inorganic reinforcing fibers provided with a multi-layer protective coating comprising a boron nitride sublayer and an oxide-based overcoating of alumina or synthetic mica, and fiber-reinforced ceramic matrix composite products incorporating the protectively-coated fibers, are disclosed. The fibers offer improved oxidation resistance and good compatibility with oxide-based glass and glass-ceramic composite matrix materials.

BACKGROUND OF THE INVENTION

The present invention relates to inorganic composite materials of theclass generally known as fiber-reinforced ceramic matrix compositematerials. The invention includes inorganic reinforcing fibers for suchcomposites incorporating multi-layer protective coatings, and improvedceramic composites incorporating such protective coatings.

Fiber-reinforced ceramic matrix composites comprising glass-ceramicmatrices are well known. U.S. Pat. No. 4,615,987 discloses siliconcarbide fiber reinforced glass-ceramic composites wherein the matrixconsists of an alkaline earth aluminosilicate glass-ceramic composition.Similar silicon-carbide-reinforced composites wherein the matrixconsists of a barium-modified magnesium aluminosilicate glass-ceramicare reported in U.S. Pat. No. 4,589,900, while U.S. Pat. No. 4,755,489discloses SiC-reinforced glass-ceramics wherein the glass-ceramic matrixcontains excess Al₂ O₃ and consists predominantly of anorthite incombination with mullite or alumina.

Prospective uses for fiber-reinforced ceramic matrix composites such asdescribed in these and other prior patents and literature include use asa structural element in high temperature environments such as heatengines. Thus the materials to be employed must not only exhibit goodstrength and toughness at ambient temperatures, but must also retainthose desirable physical properties at the elevated temperaturesencountered in the operating environment. Temperatures in the range of700°-1000° C. and highly oxidizing conditions (due to thehigh-temperature activity of oxygen) are considered representative ofsuch operating conditions.

An important problem which has been identified in silicon carbidereinforced ceramic matrix composites in this temperature regime is thatof high temperature embrittlement. After exposure to temperatures in theoperation ranges desired, these initially tough materials become brittleand subject o sudden catastrophic breakage instead of the more gradualfailure typical of the original material. While the exact mechanism oembrittlement has not been fully explained, oxidative deterioration ofthe fiber-matrix interface is the probable cause. See, for example, R.L. Stewart et al., "Fracture of SiC Fiber/Glass-Ceramic Composites as aFunction of Temperature," in Fracture Mechanics of Ceramics, R. C. Bradtet al. Ed., Volume 7, pages 33-51, Plen (New York) 1986.

It is known to provide coatings on fiber reinforcement to beincorporated in composite materials in order to modify the behavior ofthe materials or the fibers therein. For example, U.S. Pat. No.4,642,271 suggests boron nitride coatings for silicon carbide and otherfibers for incorporation in ceramic matrix materials such as SiO₂, ZrO₂,mullite, and cordierite. In SiO₂ systems, high-temperature strength antoughness were improved utilizing SiC reinforcing fibers coated with BN,although this effect was not observed in all matrix systems.

Other coating systems and coating/matrix combinations are also known.U.S. Pat. No. 4,276,804, for example, describes carbon fibers coatedwith a metal oxide film intended to improve fiber adhesion and wettingby a molten metallic matrix material. U.S. Pat. No. 4,397,901 describesa composite wherein a woven or non-woven fiber substrate of carbonfibers is provided with a multi-layer carbon/silicon/silicon carbidecoating to provide a composite article resistant to corrosiveconditions. U.S. Pat. No. 4,405,685 describes a similar coating systemfor carbon fibers wherein an inner coating of carbon and a selectedmetal carbide with an outer coating of the metal carbide are used. Thiscoating system is intended to provide enhanced fiber protection forfibers to be embedded in ceramic or particularly metal matrix materials.

U.S. Pat. No. 4,481,257 discloses silicon carbide monofilaments coatedwith boron or boron carbide and exhibiting improved strength and bondingwhen used with metal or epoxy matrix materials. U.S. Pat. No. 4,485,179describes the use, in a ceramic matrix composite comprising siliconcarbide fibers, of an agent added to the matrix to reduce interactionwith the silicon carbide fibers. Tantalum or niobium compounds areuseful for this purpose.

While the foregoing patents and literature indicate a general interestin the development of coatings for fibers to be employed for thereinforcement of composite glass, metal and ceramic materials, theproblem of embrittlement of ceramic matrix composites comprising siliconcarbide or other fibers remains.

It is therefore a principal object of the present invention to provide afiber-reinforced ceramic matrix composite comprising inorganicreinforcing fibers which exhibits improved resistance to embrittlementunder adverse high temperature conditions.

It is a further object of the invention to provide a method for makingsilicon carbide-reinforced glass-ceramic matrix composites whichprovides products of improved strength and/or toughness at hightemperatures.

It is a further object of the invention to provide a novel and improvedprotective coating system for inorganic fibers utilized for ceramicmatrix reinforcement, particularly glass-ceramic matrix reinforcement,and coated fibers incorporating the coating system.

Other objects and advantages of the invention will become apparent fromthe following description thereof.

SUMMARY OF THE INVENTION

The present invention is founded upon the discovery of a new multi-layercoating system for inorganic reinforcing fibers used for thereinforcement of ceramic materials. The multi-layer coatings of theinvention provide an adherent and cohesive protective layer which iseffective to preserve the inherent strength of reinforcing fibers suchas silicon carbide fibers, while being physically and chemicallycompatible with both the fibers and common ceramic matrix materialsincluding refractory glass and glass-ceramics.

In a first aspect, then, the invention includes a coated inorganic fibermaterial having a multi-layer protective surface coating, wherein themulti-layer surface coating comprises a boron nitride sublayer and anoxidic overcoating adhering to the sublayer. The boron nitride sublayerhas a composition consisting essentially, in weight percent, of about75-90% BN, 0-10% O and 0-14% C, while the oxide overcoating consists atleast predominantly of an oxidic species selected from the groupconsisting of alumina and synthetic mica.

In another aspect, the invention includes a fiber-reinforced ceramicmatrix composite article exhibiting high-temperature strength andtoughness comprising a ceramic matrix selected from the group consistingof glasses and glass-ceramics in which are disposed inorganicreinforcing fibers having an improved multi-layer protective coatingthereon. The multi-layer protective coating includes a boron nitridesublayer having a composition consisting essentially, in weight percent,of about 75-90% BN, 0-10% O and 0-14% C, and an oxidic overcoatingconsisting predominantly of an oxidic species selected from the groupconsisting of alumina and synthetic mica.

In a third aspect, the invention includes a method for treatinginorganic fiber to improve the oxidation resistance and bondingcharacteristics thereof within an encapsulating ceramic matrix materialwhich comprises the step of depositing onto the surface of the fiber amulti-layer protective coating comprising a boron nitride sublayer andan oxidic overcoating. The boron nitride sublayer has a compositionconsisting essentially, in weight percent, of about 75-90% BN, 0-10% Oand 0-14% C, and the oxide overcoating consists at least predominantlyof an oxidic species selected from the group consisting of alumina andsynthetic mica.

In the preferred method, the boron nitride sublayer is deposited on thesurfaces of the inorganic fiber by chemical vapor deposition, while theoxidic alumina or mica overcoating is deposited on the fiber by liquidcoating followed by pyrolysis. A liquid coating comprising one or moreorganometallic precursors of an oxidic compound selected from the groupconsisting of alumina and synthetic mica is applied to the surface ofthe fiber, and the fiber is then heat treated to convert the precursorsto the selected oxidic compound.

The application of an alumina or mica overcoating to BN-coated inorganicfibers appears to significantly enhance the performance of the BNcoating or film. Without being bound by any particular explanation as tothe mode of operation of the invention, it is presently believed thatthe oxidic overcoating protects the relatively soft BN subfilm duringcomposite processing and/or acts as a getter to purify the BN subfilmduring hot pressing and/or protects the BN subfilm from harmful chemicalinteraction with the matrix during hot pressing.

Alumina, for example, has a very low diffusion coefficient for oxygenand in the form of a continuous coating could function as an excellentoxygen diffusion barrier. It is also known to be reactive with B₂ O₃ toform a refractory Al₁₈ B₄ O₃₃ mullite phase, or a moderately refractoryAl₄ B₂ O₉ phase. In this way alumina could act as a boron oxide getterat the coating interface. This would have two effects: it could driveoxygen from the coating, purifying the BN, and could tie up thelow-melting B₂ O₃ phase, resulting in a more refractory interface.

Mica overcoatings, on the other hand, offer the potential for a secondpoint of fiber debonding that imparts toughness to the compositematerial. Such a configuration is thought to reduce the exposure of theBN underlayer to reactive environments. In any case the advantages ofthe protective coating system of the invention in terms of the improvedhigh temperature properties thereof will be apparent from the followingdescription.

DETAILED DESCRIPTION

The invention is not believed to be limited in its application to anyparticular types of inorganic reinforcement fibers or ceramic matrixmaterials. In the case of the fibers, the presently preferredreinforcement materials are silicon carbide and silicon oxycarbidefibers, which are known to be subject to high temperature embrittlementif not protected. Nevertheless the multi-layer coatings of the inventionwill impart good fiber-pullout behavior for improved toughness, togetherwith good coating stability, even where other fibers are employed.

Examples of alternative fibers include fibers of carbon, alumina, B₄ C,BN, zircon, mullite, or spinel. Examples of the preferred siliconcarbide-based fibers include Nicalon® silicon oxycarbide fibers,commercially available from the Nippon Carbon Co. of Tokyo, Japan.

The selection a ceramic matrix material for reinforcement with fiberscomprising multi-layer coatings in accordance with the invention islikewise not critical, although for applications requiring the best hightemperature performance, refractory alkaline earth aluminosilicateglass-ceramics are normally preferred. Such glass-ceramics aredisclosed, for example, in U.S. Pat. No. 4,615,987, and includeglass-ceramics wherein the predominant crystal phase is selected fromthe group of anorthite (CaO.Al₂ O₃.2SiO₂) and its pseudo-binaries withmullite (3Al₂ O₃.SiO₂), cordierite (2MgO.2Al₂ O₃.5SiO₂), bariumosumilite (BaO.2MgO.3Al₂ O₃.9SiO₂), albite solid solution (Na₂ O.Al₂O₃.6SiO₂), Al₂ O₃, SiO₂, CaO.SiO₂, and gehlenite (2CaO.Al₂ O₃.SiO₂.

Other refractory alkaline earth aluminosilicate glass-ceramics includethose comprising a predominant crystal phase consisting essentially oftriclinic anorthite in admixture with at least one of mullite and alphaalumina, these glass-ceramics being disclosed in U.S. Pat. No.4,755,489. Further, U.S. Pat. No. 4,464,475 discloses alkaline earthaluminosilicate glass-ceramics wherein the principal crystal phase isselected from the group consisting of barium osumilite, cordierite, andstuffed cordierite, the ions comprising the stuffing ions in the stuffedcordierite compositions being selected from the group consisting of Ba,Ca, Sr and Cs. Barium-stuffed cordierite glass-ceramics, in particular,exhibit relatively low coefficients of thermal expansion and highelastic moduli.

For somewhat less demanding applications, matrix materials comprisinglithium or zinc aluminosilicate glass-ceramics may be selected. U.S.Pat. No. 4,554,197 describes the use of glass-ceramic matrix materialsof this type, which may also contain magnesium but which are typicallyessentially free of TiO₂. These glass-ceramics are characterized by thepresence of a principal crystal phase selected from the group consistingof beta-quartz solid solution (sometimes referred to as beta-eucryptitesolid solution) and beta-spodumene solid solution.

Finally, the coated fibers of invention can be utilized for thestrengthening and/or toughening of glass matrix materials, particularlyincluding alkali-free alkaline earth aluminosilicate glasses. Theseglasses are preferably substantially free of alkali metal oxides such asNa₂ O, Li₂ O, and K₂ O, and include one or more alkaline earth metaloxides selected from the group consisting of CaO, MgO, SrO and BaO.

The application of a boron nitride coating to provide a subfilm on theselected fiber reinforcement material can be carried out by conventionalchemical vapor deposition techniques. As noted in U.S. Pat. No.4,642,271, a fiber material such as Nicalon® silicon oxycarbide fibertow may be coated in a vacuum chamber by contact with mixtures of boronand nitrogen source gases such as BCl₃, borazine and ammonia at anelevated temperature at which source compound decomposition and BNformation will occur directly on the surfaces of the fibers.

Alternative deposition procedures, including those involving thedeliberate introduction of dissolved carbon (graphite) into the coatingfrom a carbon source compound provided in the source gas mixture, may ofcourse be employed. Any method providing adherent BN sublayersconsisting at least predominantly of BN, e.g., at least about 70% BN byweight, is suitable. Additional constituents which may be present inthese sublayers without adversely affecting coating performance includeup to about 14% carbon and up to about 10% oxygen by weight.

A relatively large number of synthetic mica compositions known in theliterature may be used for the successful mica overcoating of BN-coatedfibers in accordance with the invention. The preferred mica overcoatingsare synthetic fluormicas, most preferably synthetic potassiumtetrasilicic fluormica (KMg₂.5 Si₄ O₁₀ F₂) or potassium fluorphlogopitemica (KMg₃ AlSi₃ O₁₀ F₂). However, other synthetic micas, includingother normal fluorphlogopites, boron fluorphlogopites, sub-potassicfluorphlogopites, lithia fluormicas, and alkaline earth disilicic andtrisilicic fluormicas, may alternatively or additionally be used.

Oxidic fluormica compounds for fiber overcoating may be prepared by thecontrolled crystallization of glasses, as disclosed in U.S. Pat. Nos.3,689,293, 3,732,087 and 3,756,838, and may be applied to fibers asprecursor glass or crystalline powders. Alternatively, inorganic micasols comprising these or other micas may be prepared as disclosed inU.S. Pat. No. 4,239,519 and the sols used to coat the fibers.

As previously noted, however, the deposition of an oxidic alumina ormica overcoating to BN-coated reinforcing fibers in accordance with theinvention is most preferably accomplished through the use of solutionsor liquid sols of organometallic source compounds for the overcoatings.While other methods of deposition such as vapor deposition, powdercoating, gel coating or the like may alternatively be used for many ofthese materials, solution or sol coating insures minimal chemical orphysical damage to the BN sublayer and offers excellent compositioncontrol, fiber coverage, and coating homogeneity for the more complexmica overcoating materials.

The preferred organometallic source compounds for the solution coatingof fibers as described are the alkoxide compounds of aluminum, silicon,boron and the alkali and alkaline earth metals commonly included in themica overcoatings. However, other organometallic compounds formingstable solutions or sols in aqueous or non-aqueous media mayalternatively be used.

Alkoxide solutions can readily be converted to alkoxide gels uponexposure to gelling agents such as water, with the mica crystals thenbeing developed directly in the gels by an appropriate heat treatment.Most preferably, the crystals are developed in situ on the fibers byfirst coating the fibers with the solutions, then inducing gelation ofthe solutions on the fibers, and finally heat-treating the fiberscomprising the gelled coatings to convert the coatings to crystallinelayers.

As known in the art, coated reinforcing fibers such as silicon carbidefibers can conveniently be incorporated into glass-ceramic matrixmaterials such as above described if the matrix materials are providedas glasses in particulate or powdered form. Such particulate matrixmaterials may readily be produced from glasses by drigaging, grindingand/or milling, with the glass powders thus produced being readilyapplied to the fibers in the form of liquid suspensions of the powders.Typically, these suspensions comprise dispersants and binderconstituents in addition to the glass powders, and are applied byspraying or immersion of fibers or fiber tows or mats into thesuspensions.

Fiber mats or tows impregnated with powdered glass as described may thenbe pressed or wound onto drums to provide green sheets or prepregs ofthe glass-impregnated fibers. These may then be stacked, if desired, andheated to achieve burnout of organics present in the coating vehicle.

Consolidation of the green prepregs or stacks thereof is typicallyaccomplished by a hot pressing process during which the temperature israised above the softening temperature of the glass and pressure isapplied to eliminate voids in the material and produce a densecomposite. In the case of glass-ceramic matrix materials,crystallization of the matrix material to effect conversion to aglass-ceramic matrix is usually achieved concurrently with consolidationin the course of the hot pressing process.

The invention may be further understood by reference to the followingexamples which set forth specific illustrative embodiments thereof.

Example 1 Coated Fibers

Inorganic fibers are first selected for treatment by a multi-layercoating process. The fibers selected are commercially available Nicalon®NLM 202 fiber tows, each tow consisting of approximately 500 SiC(silicon oxycarbide) fibers of substantially cylindrical cross-section.The individual fibers have diameters of about 10-15 microns, and assupplied include a polyvinyl acetate sizing material which is removedfrom the fiber surfaces by pyrolysis prior to the actual deposition ofthe multi-layer coating.

The selected fiber tows are provided with a boron nitride base layer byconventional commercially available processing. Processing iscommercially available from the Synterials Company of Herndon, Va., USA.Fibers comprising a BN coating approximately 0.2 microns in thickness,coated over a 10 minute interval under a vacuum of 0.3 torr at 960° C.,using reactant flow rates of 4.31 g/min BCl₃ and 0.77 g/min NH₃, areselected for further processing. The BN coatings include oxygen and freecarbon impurities, but have compositions comprising in excess of 75% BNby weight.

Alkoxide sols for the application of alumina and synthetic fluormicaovercoatings to the BN-coated fibers are next prepared. For a potassiumphlogopite mica overcoating sol, 1.66 g (0.0237 moles) of potassiumethoxide, 8.13 g (0.0711 moles) of magnesium ethoxide, 4.84 g (0.0237moles) of aluminum isopropoxide, and 14.78 g (0.071 moles) of siliconethoxide are measured into a flask. To this mixture is added 375 ml of2-methoxy ethanol and 25 ml of concentrated HNO₃. The alkoxide solutionthus provided is stirred and warmed until all of the componentsdissolve.

Next, 1.76 g (0.0303 moles) of NH₄ F.HF is dissolved in 92 ml ofmethanol and 8 ml of HNO₃ in a separate flask. This methanol solution isthen added to the alkoxide solution while the latter is being stirredand warmed. The alkoxide solution is clarified by this addition to yielda yellow colored, clear, non-viscous sol which thereafter is warmed andstirred for 1 to 2 hours.

Finally, 300 ml of 2-methoxy ethanol is added to the sol to yield afinal volume of about 800 ml of product containing the precursor toabout 0.0237 moles of mica. Thus this sol provides about 1.25 g ofpotassium fluorphlogopite mica for each 100 ml of solution.

For a potassium tetrasilicic fluormica overcoating sol, magnesium,silicon and potassium alkoxides are weighed into a flask in a glove boxunder nitrogen. Silicon tetraethoxide (12.2 g), magnesium diethoxide(4.18 g), and potassium methoxide (1.03 g) are dissolved in 286 ml of2-methoxy ethanol together with 14 ml of concentrated HNO₃ (69% acidsolution). The resulting mixture is refluxed under flowing nitrogen gasto provide a homogeneous solution.

Next 1.07 g of NH₄ HF₂ is dissolved in 96 ml of methanol and acidifiedwith 4 ml of concentrated HNO₃, the resulting mixture being addeddropwise to the refluxing solution. This addition provides a clearyellow solution with no significant viscosity increase over that of theoriginal 2-methoxy ethanol solvent. Little or no precipitation ofcrystalline mica occurs. The solution thus provided is then converted toa clear tetrasilicic potassium fluormica gel by allowing air hydrolysisof the solution to occur in an open container coincident with theevaporation of excess solvent from the solution.

The crystalline mica products derived from each of the mica sols or gelsabove described yield x-ray diffraction powder patterns manifestingstrong characteristic sheet silicate features. Some residual glass isshown, but no crystalline impurities are readily detectable.

For an alumina overcoating sol, 8 g of aluminum isopropoxide is weighedout in a glove box and combined with 196 mls of ethanol in a 500 mlboiling flask. The mixture is then acidified by the addition of 10 ml ofconcentrated nitric acid. After refluxing for approximately 2 hour, ahomogeneous Al₂ O₃ sol which is clear and colorless to slightly yellowin color with slight white residue is produced.

Any of the three alkoxide solutions or gels produced as above describedcan conveniently be used to apply solution or gel coatings toBN-precoated fibers for alumina or mica overcoating. To apply the threeovercoatings, BN-coated fibers are drawn through the selected sol orgel, preliminarily dried at 150° C. to remove excess solvent, fired and275° C. to remove organic species, and then collected on a takeup spool.For the alumina and potassium fluorphlogopite mica preparations, thisprocess is repeated once to double the thickness of the appliedovercoating.

In the case of the alumina overcoating, no further firing is needed.However, for the fibers overcoated with the mica preparations, thecoated fibers are transferred to a ceramic spool for further heattreatment to develop the mica crystalline phase in each coating. Thewound fibers are heated at a rate of 10° C./min to 490° C., held for 20minutes at that temperature, and finally cooled to room temperature. Areducing atmosphere of forming gas (92% nitrogen, 8% hydrogen) at a flowrate of 10 l/min is provided in the heating chamber during thistreatment.

Examination of the coated fibers produced in accordance with the aboveprocess indicates that smooth, adherent multi-layer coatings of BN andovercoated alumina or mica are obtained. The alumina sol provides fullycontinuous overcoating coverage while the mica sol/gels provide coverageof at least 70-80% of the fiber surface.

Example 2

Ceramic Composite Fabrication

Silicon oxycarbide fiber tows comprising three different types ofmulti-layer coatings produced as described in Example 1 above areselected for incorporation into a glass-ceramic matrix composite articleFiber tows incorporating each of the three coating systems are firstcombined with a powdered alkaline earth aluminosilicate glass precursorfor a glass-ceramic matrix material The precursor consists of a powderedcalcium aluminosilicate glass having an oxide composition, in weightpercent, of about 40.8% SiO₂, 39.7% Al₂ O₃, 19.0% CaO, and 0 5% As₂ O₃,and having an average particle size of about 10-15 microns for themilled glass. The composition of this glass is such that it can beconverted to a highly crystalline anorthite glass-ceramic matrixmaterial upon suitable heat treatment

A suspension of the powdered glass useful for impregnating the siliconcarbide fiber tows with the matrix powder is prepared by combining thepowdered glass with a liquid vehicle comprising an alcohol/water solventmixed with a polyvinyl butyral binder and a dispersant in conventionalfashion. The tows are then continuously drawn through this suspensionand wound onto a drum to form a cylindrical fiber layup.

After drying, the glass-impregnated fiber winding is cut from the drumand stretched flat, and preform sheets are cut from the flattenedwinding for subsequent processing. Suitably, the cut sheet samples arepreliminarily heated to evaporate residual solvent materials therefromand then processed through a burnout step to remove organic binders.Burnout comprises heating the samples for two hours in nitrogen or airat approximately 550°-650° C.

Panels of composite material are prepared from these preform sheets bystacking and consolidating the sheet stacks with heat and pressure.Stacks of twelve sample sheets in fiber-parallel alignment areconsolidated by hot pressing the stacks under nitrogen to temperaturesin the range of about 1200°-1360° C. at pressures in the range of about1500-3000 psi. This treatment removes voids from the material andconverts the glass powder matrix to a dense crystalline glass-ceramicmatrix wherein the principal crystal phase is anorthite. The hotpressing does not appear to significantly degrade the multi-layerprotective fiber coatings.

Composite samples produced in accordance with the above-describedprocedure are evaluated for resistance to embrittlement by a strengthtesting procedure during which they are tested in flexure for microcrackyield stress and ultimate flexural strength, both at ambient temperature(25° C.) and at elevated test temperatures up to 1300° C. In the hightemperature flexural tests, the samples undergo deformation,microcracking, and ultimate failure under conditions where almostimmediate sample embrittlement and weakening occur if unprotectedsilicon carbide fibers are used. Thus improvements in fiber protectioncan readily be evaluated in these tests.

In addition to low high-temperature strength, the extent ofembrittlement of composites exposed to this testing is further indicatedby changes in fracture habit. Tough, non-brittle samples show fracturecharacterized by fiber pullout from the matrix (a so-called fibrousfracture habit), whereas woody and, especially, brittle fracture habitssuggest increasing levels of embrittlement. Woody fracture surfacesdisplay some crack propagation parallel to the stress axis, indicatinglocalized shear failure but without fibrous pullout, whereas brittlefracture surfaces display merely planar fracture surfaces typical, forexample, of conventional glass.

Typical results of the flexural testing of composite panel materialsproduced in accordance with Example 2 are recorded in Table I below.Included in the Table for each of several numbered composite samplestested are an identification of the overcoating preparation employed toprotect the silicon oxycarbide reinforcing fibers in that sample,whether alumina (Al₂ O₃), potassium fluorphlogopite mica (KF mica), orpotassium tetrasilicic fluormica (KT mica), and the hot pressing (HotPress) conditions of peak temperature and pressure used for theconsolidation and crystallization of the materials.

For each temperature (Test Temp.) used for flexural testing of thesamples, test data including the microcrack yield stress (σ_(mcy)), themicrocrack yield strain (ε_(mcy) or sample elongation at the point ofmicrocracking), ultimate flexural strength (σ_(ult)), and the strain orelongation at the ultimate failure point (ε_(ult)) are reported. Allflexural testing is done on the strong or fiber reinforcement axis ofthe samples.

Finally, the fracture behavior observed for each of the samples testedis reported. The reported fracture data includes an indication of theprincipal failure mode(s) for each sample, whether in tension (T),compression (C), shear (S) or deformation (Def.), and the predominantfracture habit(s) observed, whether fibrous, woody, or brittle.

                  TABLE I                                                         ______________________________________                                        Sample                Test       .sup.σ mcy                                                                    .sup.ε mcy                     No.     Hot Press     Temp.(°C.)                                                                        Ksi   (%)                                    ______________________________________                                        1       1360 °C./1500 psi                                                                     25        17.5  0.14                                   Al.sub.2 O.sub.3      1000       11.3  0.11                                   2       1200 °C./1500 psi                                                                     25        19.3  0.15                                   Al.sub.2 O.sub.3      1000       21.6  0.22                                   3       1200 °C./1500 psi                                                                     25        35.8  0.23                                   Al.sub.2 O.sub.3      1000       29.6  0.27                                                         1200       12.7  0.17                                                         1300       9.3   0.23                                   4       1250 °C./1500 psi                                                                     25        34.0  0.21                                   Al.sub.2 O.sub.3      1000       15.8  0.14                                   5       1200 °C./1500 psi                                                                     25        42.3  0.25                                   Al.sub.2 O.sub.3      1000       17.0  0.12                                                         1200       14.2  0.13                                   6       1250 °C./1500 psi                                                                     25        42.4  0.26                                   Al.sub.2 O.sub.3      1000       13.3  0.26                                                         1200       7.6   0.15                                   7       1200 °C./1500 psi                                                                     25        41.7  0.25                                   KF mica               1000       15.2  0.16                                                         1200       6.6   0.08                                   8       1250 °C./1500 psi                                                                     25        46.5  0.27                                   KF mica               1000       19.0  0.16                                                         1200       8.4   0.11                                   9       1250 °C./1500 psi                                                                     25        19.8  0.14                                   KT mica               1000       13.7  0.14                                   10      1250 °C./3000 psi                                                                     25        14.7  0.11                                   KT mica               1000       15.4  0.15                                   11      1200 °C./1500 psi                                                                     25        39.4  0.23                                   KT mica               1000       14.2  0.15                                                         1200       8.5   0.12                                                         1300       5.1   0.12                                   12      1250 °C./1500 psi                                                                     25        22.7  0.14                                   KT mica               1000       14.7  0.14                                   ______________________________________                                        Sample  Test     .sup.σ ult                                                                       .sup.ε ult                                                                  Fracture                                      No.     Temp.    Ksi      (%)   Mode/Character                                ______________________________________                                        1        25      81.1     0.97  T,S; fibrous                                  Al.sub.2 O.sub.3                                                                      1000     48.6     0.59  T,S; fibrous                                  2        25      87.0     0.90  T,S; fibrous                                  Al.sub.2 O.sub.3                                                                      1000     107.4    1.22  S,C,T; fibrous                                3        25      102.5    0.86  T,S; woody & fibrous                          Al.sub.2 O.sub.3                                                                      1000     101.6    1.08  T,S; woody & fibrous                                  1200     70.7     1.34  Def,S,T; brittle-woody                                1300     42.4     1.78  Def,S; brittle-woody                          4        25      133.2    1.13  T,S; fibrous to woody                         Al.sub.2 O.sub.3                                                                      1000     92.5     1.00  T,S; fibrous to woody                         5        25      124.6    0.85  T,S; brittle to fibrous                       Al.sub.2 O.sub.3                                                                      1000     130.4    1.26  T,S; woody to fibrous                                 1200     71.7     0.79  T,S; woody to fibrous                         6        25      119.7    0.84  T,S; woody to fibrous                         Al.sub.2 O.sub.3                                                                      1000     111.3    0.98  T,S; woody to fibrous                                 1200     70.9     0.77  T,S; woody to fibrous                         7        25      136.1    0.89  T,S; fibrous                                  KF mica 1000     92.2     1.01  T,S; fibrous-woody                                    1200     54.2     0.93  T,S; fibrous-woody                            8        25      124.4    0.87  T,S; fibrous                                  KF mica 1000     83.8     0.84  T,S; fibrous                                          1200     46.8     0.80  T,S; fibrous-woody                            9        25      137.2    1.29  T,S; fibrous                                  KT mica 1000     73.3     1.03  T,S; fibrous                                  10       25      109.7    0.98  T,S; fibrous                                  KT mica 1000     69.4     0.84  T,S; fibrous-woody                            11       25      112.2    0.83  T,S; brittle-woody                            KT mica 1000     74.5     0.86  T,S; brittle-woody                                    1200     36.7     0.85  Def,S; brittle-woody                                  1300     14.1     0.73  Def,S; brittle-woody                          12       25      86.5     0.71  T,S; brittle-woody                            KT mica 1000     89.9     1.00  T,S; brittle-woody                            ______________________________________                                    

The data reported in Table I above indicate that compositesincorporating fibers with multi-layer coatings in accordance with theinvention provide high temperature oxidation resistance which issuperior to composites incorporating unprotected fibers, as well as tocomposites incorporating fibers with BN protective coatings alone. TableII below reports performance de, as for control samples having thematrix composition of the samples reported in Example 2, but comprisinguncoated silicon oxycarbide fibers or silicon oxycarbide fibers coatedwith BN alone.

    ______________________________________                                        TABLE II - Prior Art Samples                                                           Test                                                                 Sample/  Temp.        .sup.σ mcy                                                                      .sup.ε mcy                              Coating  (°C.) (Ksi)   (%)                                             ______________________________________                                        Control   25          28-40   0.2-0.4                                         (none)   1000         28-32   0.15-0.25                                       Control   25          25-40   0.2-0 4                                         (BN)     1000         18-35   0.2-0.3                                                  1200         10-20   0.15-0.3                                                 1300          5-10   0.1-0.2                                         ______________________________________                                        TABLE II - Concluded                                                                  Test                                                                  Sample/ Temp.    .sup.σ ult                                                                       .sup.ε ult                                                                    Fracture                                    Coating (°C.)                                                                           Ksi      (%)     Mode-Habit                                  ______________________________________                                        Control  25       80-120  0.6-1.4 Fibrous                                     (none)  1000     36-40    0.25-0.4                                                                              Brittle/woody                               Control  25      70-90     0.7-1.25                                                                             Fibrous                                     (BN)    1000      70-100  0.7-1.2 Fibrous/woody                                       1200     30-50    0.6-1.2 Fibrous/Def.                                        1300      5-15    0.4-0.6 Def.                                        ______________________________________                                    

Flexural test data such as reported in the above Tables suggest distincthigh-temperature performance advantages for the composites provided withmulti-layer protective coatings in accordance with the invention. Thusalthough the average room temperature properties for the inventivecomposites are comparable to those of the prior art compositescontaining BN-coated fibers, flexural performance for the lattercomposites at 1000° C. include ultimate strengths averaging 75-76 Ksiand ultimate strains averaging about 0.65%.

The inventive composites with either alumina or mica overcoating layersdisplay significant high temperature strength improvements. Micaovercoated samples tested at 1000° C. have ultimate flexural strengthsaveraging approximately 74.5 Ksi with strains to failure averaging0.86%. The alumina overcoated samples exceed even this performance withaverage ultimate strengths of 101.6 Ksi and strains to failure of 1.08%.Thus the latter composites show virtually no degradation in flexureproperties from room temperature to 1000° C.

Scanning Auger analyses of multi-layer coatings on fibers provided inaccordance with the invention suggest that the mica and aluminaovercoating layers in these coatings may help to stabilize BN sub-layercomposition. Analysis of oxygen content in the BN sub-layers indicatethat average oxygen impurity levels are maintained at about 10% byweight. This is below levels frequently reached in unprotected BN-coatedfibers and suggests that a good level of protection of the BN sub-layersis being provided by the oxidic overcoatings.

Composites fabricated using overcoated fibers in accordance with theinvention can also exhibit excellent strength retention afterprestressing and subsequent air oxidation at elevated temperatures of650° C. and 1000° C. Composites with alumina overcoatings displayretained room temperature tensile strengths of 75 to 90% of the originalstrength after prestressing to 1.5 times the microcrack yield point(i.e., 35-50 Ksi), followed by oxidation for 10 hours at 1000° C.Composites with mica overcoatings have demonstrated retained strengthsof 50-90% of their original strength after similar prestressing andexposure.

Similar damage accumulation tests with prior art samples yield less than20% strength retention in BN-coated samples. In fact, samples with BNcoatings alone generally sustain an exposure loss of 20% in strengthindependent of prestress level.

As is evident from the foregoing description, then, the performance ofthe composites of the invention offers improved prospects for securinghigh temperature oxidation resistance sufficient for the design ofload-bearing parts for heat engine applications.

While the invention has been particularly described above with respectto specific materials and specific procedures, it will be recognizedthat those materials and procedures are presented for purposes ofillustration only and are not intended to be limiting. Thus numerousmodifications and variations upon the compositions and processesspecifically described herein may be resorted to by those skilled in theart within the scope of the appended claims.

We claim:
 1. A fiber-reinforced ceramic matrix composite articleexhibiting high-temperature strength and toughness comprising a ceramicmatrix selected from the group consisting of glasses and glass-ceramicsin which are disposed reinforcing inorganic fibers having a protectivecoating thereon, wherein:the protective coating is a multi-layer surfacecoating comprising (i) a boron nitride sublayer having a compositionconsisting essentially, in weight percent, of about 75-90% BN, 0-10% Oand 0-14% C, and (ii) an oxide overcoating consisting predominantly ofan oxidic species selected from the group consisting of alumina andsynthetic mica.
 2. A fiber reinforced ceramic matrix composite articlein accordance with claim 1, wherein the ceramic matrix is selected fromthe group of refractory alkaline earth aluminosilicate glass-ceramics.3. A fiber reinforced ceramic matrix composite article in accordancewith claim 2 wherein the ceramic matrix is an alkaline earthaluminosilicate glass-ceramic matrix wherein the predominant crystalphase is selected from the group of anorthite (CaO.Al₂ O₃.SiO₂) and itspseudo-binaries with mullite (3Al₂ O₃.SiO₂), cordierite (2MgO.2Al₂ O₃.5SiO₂), barium osumilite (BaO.2MgO.3Al₂ O₃.9SiO₂), albite solid solution(Na₂ O.Al₂ O₃.bSiO₂), Al₂ O₃, SiO₂, CaO.SiO₂, and gehlenite (2CaO.Al₂O₃.SiO₂).
 4. A fiber reinforced ceramic matrix composite article inaccordance with claim 2 wherein the ceramic matrix is an alkaline earthaluminosilicate glass-ceramic matrix wherein the predominant crystalphase consists essentially of triclinic anorthite in combination with atleast one of mullite and alpha alumina.
 5. A fiber reinforced ceramicmatrix composite article in accordance with claim 2 wherein the ceramicmatrix is an alkaline earth aluminosilicate glass-ceramic matrix whereinthe principal crystal phase is selected from the group consisting ofbarium osumilite, cordierite, and stuffed cordierite, the ionscomprising the stuffing ions in the stuffed cordierite compositionsbeing selected from the group consisting of Ba, Ca, Sr and Cs₂ O.
 6. Afiber reinforce ceramic matrix composite article in accordance withclaim 1 wherein the ceramic matrix is a lithium, or zinc aluminosilicateglass-ceramic matrix wherein the principal crystal phase is selectedfrom the group consisting of beta-quartz/beta-eucryptite solid solutionand beta-spodumene solid solution.
 7. A fiber reinforced ceramic matrixcomposite article in accordance with claim 1 wherein the ceramic matrixis an alkali-free alkaline earth aluminosilicate glass including atleast one alkaline earth metal oxide selected from the group consistingof CaO, MgO, SrO and BaO and being essentially free of Na₂ O, Li₂ O, andK₂ O.
 8. A fiber reinforced ceramic matrix composite article inaccordance with claim 1 wherein the reinforcing inorganic fibers arefibers having a composition selected from the group consisting ofsilicon carbide, silicon oxycarbide, carbon, alumina, B₄ C, BN, zircon,mullite and spinel.
 9. A fiber reinforced ceramic matrix compositearticle in accordance with claim 1 wherein the reinforcing inorganicfibers are silicon oxycarbide fibers.